The adverb has 3 meanings:
Meaning #1:
with little or no delay
Synonyms: quickly, quick
Derived from adjective: prompt (meaning #3)
Meaning #2:
in a punctual manner
Synonyms: readily, pronto, without delay
Derived from adjective: prompt (meaning #1)
Meaning #3:
at once (usually modifies an undesirable occurrence)
Synonym: right away




